This invention relates to a variable displacement hydraulic machine capable of working as an engine or as a pump, in particular for a hybrid vehicle, as well as a power train and a hybrid vehicle equipped with such hydraulic machine.
Hybrid vehicles comprise in general a combustion engine which provides the main motorization of the vehicle and an additional motorization using energy that can be stored, such as electric or hydraulic energy, to optimize the combustion engine operation.
Hybrid vehicles using hydraulic energy, comprise a hydraulic machine connected to drive wheels of the vehicle, capable of operating as a pump to charge the hydraulic pressure accumulators which store energy, or as an engine to deliver mechanical power to the drive wheels while drawing energy stored in the accumulators.
In particular, during braking, one can use the hydraulic machine as a pump to recover and store the kinetic energy of the vehicle; this energy is subsequently returned by the hydraulic machine operating as an engine for driving the vehicle. This use of stored hydraulic power enables optimizing the combustion engine operation, and to reduce its fuel consumption and the polluting gas emissions. The hydraulic energy storage also permits driving in hydraulic mode or zero emission mode “ZEV” during which the vehicle does not emit polluting gases, with the combustion engine being shut down.
A type of known variable displacement hydraulic machine comprises a rotating cylinder comprising axial pistons distributed as a crown around the axis, which are connected to a plate which is located in a substantially transverse plane. By controlling or adjusting the slope of the plate with respect to the transverse plane, one modifies the stroke of the pistons that can go from a zero stroke with the plate in the transverse plane, resulting in a zero displacement, to a maximum stroke with the strongest plate slope, resulting in the maximum displacement.
A known hydraulic machine made according to a variant, shown for example in document WO-A1-2003/058035, comprises a cylinder with a series of axial bores in which floating or free piston rods slide. Axially on each side of the cylinder, there is a tiltable plate that supports bores in which the ends of the piston rods slide, each forming a piston.
This way, one obtains a compact hydraulic machine, comprising a single displacement that can be considerable by doubling sets of pistons on each side of the cylinder.
However, this type of hydraulic machine comprises a performance that varies greatly on the basis of the displacement actually used, of the speed and the pressure of operation. In that case, one is obligated for an operation on a vehicle that requires varied ranges of use in torque and power, to have at least two hydraulic machines comprising two different displacements, in order to use each of these machines for its optimal operating range.
Installing two hydraulic machines creates problems, in particular related to weight, overall dimensions and installation in the vehicle. In addition, this type of machine can produce noise during operation, as well as torque pulsations tied to the number of pistons.